1. Field of the Invention
This invention relates to an automatic focusing device for use in a camera which detects the focus condition of an objective lens by measuring the light from a target object passing through the objective lens, and which automatically adjusts the objective lens to its in-focus position in accordance with the detection.
2. Description of the Prior Art
Heretofore, there was proposed, for example in U.S. Pat. No. 4,185,191, a focus detecting device which detects the focusing condition, i.e. the amount and the direction of deviation of an objective lens from its in-focus position, by measuring the light coming from an object to be photographed and passing through the objective lens. Further, Japanese laid-open patent applications with laid-open numbers Tokkai-Sho 56-78811 and Tokkai-Sho 56-78823 proposed solutions to the inherent inconveniences in camera automatic focusing devices employing the above-mentioned focus detecting device. The focus detecting device disclosed in U.S. Pat. No. 4,185,191 produces an unfavourable phenomenon that, when a target moves in a plane perpendicular to an optical axis of the objective lens, the focus detecting device indicates different detections for the target object at the same distance before and after the movement. To cope with this problem, the device shown in Tokkai-Sho 56-78811 is arranged such that the focus detection for a single target object is made a plurality of times to obtain a plurality of detected signals with the objective lens being stopped. The focusing of the objective lens is adjusted in accordance with an intermediate value obtained by averaging the plural detected signal values, and thus compensating for any scatter in the values of the detected signals. This proposed device may be evaluated as advantageous from the viewpoint of accuracy of the detection, but is disadvantageous from the viewpoint of quickness of detection because it requires detecting operations of a plural of times during which the objective lens must be stationary.
On the other hand, the above-mentioned Tokkai-Sho 56-78823 proposes an automatic focusing device which detects the focusing condition and moves the objective lens towards its in-focus position. In the case where a CCD (Charge Coupled Device) is employed as the light detecting device for detecting focusing condition of the objective lens, as in the focus detection device disclosed in U.S. Pat. No. 4,185,191, it requires a certain integration time, as is shown in the art, to obtain the information of light intensity incident on all of each light-receiving or detecting element of the CCD, and further requires a given time period to read the output of each light-receiving element and to process the read-out outputs to discriminate the focusing condition. Accordingly, if the above-mentioned device employing a CCD is used to automatically adjust the objective lens being moved, the position of the objective lens at the time when the CCD detects the light intensity differs from that at the time when a signal representing the focusing condition is generated. Hence, the device shown in Tokkai-Sho 56-78823 is arranged such that the focus adjustment is made in accordance with a defocus signal which has been compensated for the amount of movement of the objective lens between the above two positions. Accordingly, the focus adjustment may be made quickly, but the problem overcome by the device proposed in the aforementioned Tokkai-Sho 56-78811 still remains. That is, as the signal representing the amount of defocus scatters at and around the true value, the single signal of the amount of defocus cannot guarantee the accuracy of the detection although compensation is made for the amount of movement of the objective lens.